1. Field of the Invention
The instant invention relates to hot-gas solder levelling machines, and more particularly to a pneumatic system for controllably lowering a printed circuit board into a pool of molten solder, stroking the board upwardly and downwardly while immersed and subsequently raising the board out of the bath past a pair of knives from which a hot gas is directed upon the board to control the thickness of solder on the circuit conductors and the like of the board and to clear the through-holes of excess solder.
2. Description of the Prior Art
In the past, various techniques have been used to remove excess solder from printed circuit boards and to clear the through-holes of excess solder. One such prior art technique involved spraying or otherwise contacting the board with molten solder and subsequently immersing the board in a pool of hot wax or oil, vibrating the board to remove the excess solder and clear the holes and then withdrawing the board to cool. This technique had associated with it certain problems relating to pollution from fumes from the hot wax or oil and thermal shock to the boards which caused the printed circuits to lose continuity in some connections.
Another prior art technique involved immersing a board in molten solder and subsequently exposing the board to a blast of hot air or other gas. This technique has proved more effective than the hot wax or oil technique, but prior art machines for accomplishing the technique were generally slow and difficult to adjust for consistent and controllably variable solder deposition thicknesses.